Thermionic device



Jan. 5, 1932.

N. RASHEVSKY THERMIONIC DEVICE Filed May 27, 192

INVENTOR A/ma/as Eashe vsK y WITNESSES: 6. 47 5&7?

' ATTORNEY Patented Jan. 1932 UNITED STATES PATENT OFFICE NICOLAS BASHEVSKY, OF W'ILKINSIBUBG, PENNSYLVANIA, ASSIGNOR T0 WESTING- HOUSE ELECTRIC & MANUFAUIUBING COMPANY, A CORPORATION OF III'QNNSYII'J- VANIA THEBMION 1G DEVICE Application filed Kay 27, 1926. Serial- No. 111,948.

' ly uniform characteristics.

In the construction of cathodes, it is highly desirable that elements which have a low work function be used. This work function or, as it is sometimes known, electron aflinity, is expressed in terms of electrical potential or volts. The work function 4 is expressed in terms of Richardsons constant D by the relation:

thermionics, as on it depends the thermionic.

' current which can be obtained from any particular type of cathode at any given temperature and is characteristic of the cathode substance.

The smaller we make the larger in general the thermionic current that can be obtained with a given cathode temperature. Consequently, a cathode having a low constant will also tend to havea longer life, which is obviously desirable from the standpoint of economy. The thoriated cathode and the oxide coated cathode, now well known to those skilled in the art, are examples of cathodes which have been'so treated as to obtain a low value of work function. These cathodes have not always proved entirely satisfacto in the past, in part due to the difficulty 0 making filaments having identical properties and also to the fact that both types have a limited life and are comparatively expensive to manufacture.

The present invention contemplates using, as a cathode, an element having a low-melting point which has also a low value of work function. Among such elements, nickel seems to be one of the most desirable asit has a work function of 2.9 volts. Nickel, however, has a melting point which is too low to permit of it being used alone as a cathode material since it would be subj ect to deformation long before the desired temperature is attained. The problem, then, was to discover some practical manner in which nickel or an analogous element could be used as a cathode and could be prevented from deforming at the relatively high temperatures necessary to secure satisfactory electronic emission. This problem has been solved in an entirely satisfactory manner by employing a highly refractory substance, such as tungsten or molybdenum for the filamentary or cathode base and then applying a coating of nickel or other metal having the desired characteristics.

I have used several different methods to provide the necessary coating to practice my invention. Among the preferred forms are electroplating, the decomposition of a nickel salt in place, or deposition of the nickel from the vaporized state. The latter method,

v which has given the best results, will be more part of this application Figure 1 shows one form of an apparatus for coating a filament or other cathode element, and

Fig. 2 shows an alternative form.

In Fig. 1 is shown an evacuated container 1 having therein aproperly mounted filament 2 of tungsten, or thoriated tungsten, a cylindrical anode 3 having a lead-in 4. Surrounding the container is a coil 5- suitably connected to a source of high frequency current 6, which may be an alternator, an oscillation generator or an equivalent device. Although the anode is shown as a cylinder, any other convenient shape such as a plate may be employed, and the filament is simply shown as indicative of a general class of articles to be coated.

In Fig. 2 is illustrated a tube having a plate 3, in lieu of the cylinder shown in Fig. 1, while the other elements numbered similarly to those of Fig. 1, correspond respectively thereto. The container comprising the nickel cylinder or plate, and the filament or other element to be coated, is first carefully exhausted to obviate any danger of the nickel oxidizing. The coil 5 is then supplied with hi h-frequency current, which induces in the cylinder or plate a sufficiently heavy current to raise its temperature to the point at which the nickel begins to vaporize.

This heating is continued until the interior of the receptacle 1 is coated and becomes, to a more or less degree, opaque. This deposit on the tube walls is indicative that nickel has been thrown off from the cylinder or plate 3, and it has been found that the relative opacity of such deposit is a measure of the amount of nickel deposited on the filament as well. It is important that the filament be ke t relatively cool during this operation, in order that the nickel will remain thereon after having been deposited, and this is accomplished much more readily when the ap paratus of Fig. 2 is employed. In this view, the nickel element is shown as a plate, spaced several millimeters from the filament, which permits the filament to more efficiently radiate such heat as may be conveyed to it from the plate than is the case with a tube as shown in Fig. 1. At the same time, it is obvious that less heat is transferred froman entirely unexpected high electron emission. It is In conclusion that this extremely high emission is due to the fact that by using my new type of filament, I am enabled to heat the nickel or other element above its melting point, so that during the operation of the filament, the nickel or other coating exists thereon in a closely adherent semiliquid form. At the same time, it would appear probable that the work function of the coatmgmaterial is also reduced.

It is also probable that the increased electromc emission is in some way related to the lowering of the surface tension of the melted metallic film but I am not, at this time, prepared to state exact reasons for this nor can give a more exact theory of operation. Another peculiar fact is noted in connection begins at a lower temperature than when nickel is not present. Accordingly, by using a certain percentage of nickel with the well known thoriated filament, I am able to produce a cathode which is easier to activate, and which is extremely stable in operation.

It will thus be seen that I have produced a cathode the characteristics of which are different in many respects from those so far known and used. My cathode has a very much higher electron emission than the usual type, has a longer life and is relatively cheaper to manufacture. Numerous advantages of this type of cathode in addition to those enumerated will suggest themselves to those skilled in the art, and it is also, of course,

obvious that many other combinations of refractory carrier and low-melting point coating may be employed, all withm the range of equivalents of my invention.

The essential feature of my invention which is broadly new and of greatest importance is the use as a cathode of an element having a comparatively low melting point and low work function so arranged and carried by a refractory material that it may be heated above its melting point.

Although I have described herein only certain specific embodiments of my invention, it is not my intention to be limited thereby. No limitation except such as is necessitated by the rior art or expressed in the claims is inten ed.

I claim as my invention:

1. The method of increasin the electron emissivity of a thoriated catho e which comprises vaporizing nickel in the presence of said cathode until a thin coating of nickel is formed thereon. 2. In an electron-discharge device includmg a transparent vessel havin a cathode of re ractory material therein, t e method of increasing the electron emission of said cathode which comprises vaporizing nickel within said container until said vessel becomes opaque, whereby a thin film of nickel is formed on said cathode. v

3. In an electron-discharge device includ- -ing a transparent vessel havin a cathode of refractory material therein, t e method of increasing the electron emission of said cathode which comprises vaporizing nickel within said container while said cathode is unheated until said vessel becomes opaque, whereby a thin film of nickel is formed on said cathode.

4. A cathode comprising a cathode of refractory material having a thin film of nickel thereon, said film being in a semi-liquid form when the filament is heated to a temperature at which'it freely emits electrons.

5. A cathode comprising a thoriated filament and a thin film of nickel adsorbed thereon.

6. A cathode comprising thorium and a thin nickel film adsorbed thereon.

7. A cathode comprising a tungsten element and a thin film of nickel, said film being held to said element by adsorption.

8. In a cathode, an element composed of a refractory conductive material, and a thin film of nickel held thereon by adsorption.

.9. A cathode comprising a refractory carrier having a layer of thorium thereon, and

a thin film of nickel adsorbed on said thorlum.

10. As a step in the method of activating a thoriated cathode, the adsorption of a thin nickel film thereon.

11. A cathode comprising a cathode of refractory material having a thin film of nickel held by adsorption thereon, said film being not amalgamated with said material.

12. A cathode comprising a cathode of refractorv material having a thin film of nickel held by adsorption thereon, said film being not absorbed by said material.

In testimony whereof, I have hereunto subscribed my name this 21st day of May, 1926.

NICOLAS RASHEVSKY. 

